Paint Coverage for Wood Finishing: Solids, DFT and Transfer Efficiency
The technical coverage formula 10 × solids / DFT, why the spray gun eats a third to two thirds of it, and how to plan litres per job.
The formula the can does not print
Theoretical coverage in m²/L is 10 × volume solids % divided by dry film thickness in µm. The maths is pure geometry: one litre of paint spread at 100 µm wet covers 10 m² if it were 100% solids, and only the solids survive into the dry film. A 40%-solids 2K polyurethane at a 35 µm target therefore covers 10 × 40 / 35 = 11.43 m²/L on paper. Paint coverage runs this formula and everything downstream of it.
Transfer efficiency: where the paint really goes
Between the gun and the part, paint disappears: conventional air spray lands only 30-35% of what it sprays, airless around half, HVLP 65-90% depending on setup, brush and roller above 90%. Multiply theoretical coverage by transfer efficiency to get the practical figure: the 11.43 m²/L polyurethane becomes 7.43 m²/L through an HVLP gun at 65%. A 12 m² door in two coats takes 3.2 L through HVLP but 6.0 L through a conventional gun - the same job, double the paint bill.
Reading data sheets without being misled
Manufacturer coverage figures assume ideal application: NC lacquer at 20.6% solids and 25-38 µm quotes 6.6-7.9 m²/L, and a 2K PU at 25 µm quotes around 16 m²/L theoretical. Those numbers are attainable with brush or roller on cooperative substrates, and optimistic by half with conventional spray. Use the data-sheet mode in Paint coverage for brushing, the technical mode with a measured transfer efficiency for spraying - and believe neither until your own logbook confirms it.
Wood-specific reality checks
Wood drinks: end grain and MDF edges soak the first coat, so shops seal backs and edges before colour. Profiled surfaces - mouldings, carved panels, louvered doors - carry 10-30% more area than their footprint; add the extra in Paint coverage and quote accordingly. Moisture matters too: coating above the equilibrium moisture content for the destination climate is how finish jobs fail months later, which EMC covers in detail.
Litres, cost and the quote
Paint litres = area × coats × (1 + shape extra) / practical coverage. Priced per litre, that is the finishing line on every quotation. The productivity play is obvious from the numbers: moving a door shop from conventional spray to tuned HVLP halves paint consumption at equal film build - the film itself is unchanged, only the waste is. Film thickness targets come from the finish system's data sheet; the coverage maths above keeps the consumables line honest.